New arsenate minerals from the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. VII. Pharmazincite, KZnAsO4

Abstract

AbstractThe new mineral pharmazincite, KZnAsO4, was found in sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is closely associated with shchurovskyite, dmisokolovite, bradaczekite, arsmirandite, tilasite, johillerite, tenorite, hematite, aphthitalite and As-bearing orthoclase. Pharmazincite occurs as prismatic to acicular crystals up to 1 mm long and up to 0.03 mm thick typically combined in near parallel, radial or chaotic intergrowths, open-work aggregates or crusts up to 2 mm across. Pharmazincite is colourless to white, transparent, with a vitreous lustre. It is brittle, with a stepped fracture and a perfect cleavage parallel to [001]. Dcalc is 4.75 g cm–3. Pharmazincite is optically uniaxial (–),ω = 1.649(2), ε = 1.642(2). The Raman spectrum is reported. The chemical composition (wt.%, electron-microprobe data) is: K2O 18.98, CaO 0.14, MgO 1.20, CuO 4.41, ZnO 27.58, Fe2O3 0.15, P2O5 0.50, As2O546.67, total 99.63. The empirical formula, calculated based on 4 O apfu, is: (K0.97Ca0.01)∑0.98(Zn0.82Cu0.13Mg0.07Fe0.013+)∑1.03(As0.98P0.02)∑1.00O4.The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 6.36 (28)(111), 4.64(45)(220), 4.35(48)(002), 3.260(36)(411), 3.179(100)(222), 2.770(26)(113), 2.676(77)(600), 2.278(15)(602) and 1.710(15)(713, 115). Pharmazincite is hexagonal, a = 18.501(4), c = 8.7114(9) Å, V = 2582.4(8) Å3 and Z = 24 (single-crystal XRD data). Its space group is P63, by analogy with synthetic KZnAsO4 that has a crystal structure based upon a tetrahedral tridymite-type{ZnAsO4}– framework. It is isostructural with megakalsilite KAlSiO4. The new mineral is named for its chemical constituents.